<html><head><meta http-equiv="Content-Type" content="text/html charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><div><blockquote type="cite" class=""><div class="">On 7 Nov 2016, at 03:52, Chris Lattner <<a href="mailto:clattner@apple.com" class="">clattner@apple.com</a>> wrote:</div></blockquote><blockquote type="cite" class=""><div class=""><div class="">Introducing flow senstitive type refinement breaks this model because the type of a decl depends not just on its declaration, but on a potentially arbitrary number of imperative checks that occur between the declaration and the use. This can make it much more difficult to understand code.</div></div></blockquote><br class=""></div><div>This seems like more of a challenge for the IDE; if it can tap into the type-checker then it can determine what the narrowed type is at any given point in your code, indeed I would expect it to for the purposes of auto-completion anyway. I know you don't necessarily want a language that's reliant on good IDE support, but if you're doing something complex enough where this would become a problem and NOT using a good IDE then it seems kind of like a self-inflicted problem to me.</div><div><br class=""></div><div>Even so there's nothing in this feature that would prevent you from using shadowing if you want to, for example if a block is especially large and you feel it adds clarity.</div><div><br class=""></div><div>Actually though I'd say that for maintenance narrowing may be better, as it can clarify what a type is supposed to be at a given point, and if you break the narrowing then you'll create errors and warning that show you how you've changed the meaning of the code. Consider for example:</div><div><br class=""></div><div><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>func doSomething(value:Int?) {</font></div><div><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>if (value == nil) { value = 5 } // value is narrow to Optional<Int>.some</font></div><div><font face="Monaco" class=""><br class=""></font></div><div><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>// Lots of really important code that never causes value to become nil</font></div><div><font face="Monaco" class=""><br class=""></font></div><div><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>print(value!.description)</font></div><div><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>}</font></div><div><br class=""></div><div>Say you come back later and decide to remove the conditional at the top, now that value!, though a fair assumption at the time, can cause a runtime failure. With narrowing however you wouldn't have had to force unwrap because of the known non-nil value, but your change will break that, resulting in an error that forces you to fix it.</div><div><br class=""></div><div><br class=""></div><div>I'm still struggling how best to phrase my motivation section; so far I seem to have an increasingly large grab-bag of individual problems that type-narrowing can solve, with no way to put it more succinctly.</div></body></html>